DIRIGIBLE. Steerable, drivable; usually applied to lighter than air flying machines which may be propelled and guided.
Form and If ability to rise in the air depended merely upon a knowledge of the principle that made it possible, dirigible navi gation would undoubtedly have been accom plished many centuries ago. Archimedes estab lished the fact that a body upon floating in a fluid displaces an amount of the latter equal in weight to the body itself, and upon this theory was formulated the now well-known law that every body plunged into a fluid is subjected by this fluid to a pressure from below, equivalent to the weight of the fluid displaced by the body. Consequently, if the weight of the latter be less than that of the fluid it displaces, the body will float.
Once in the air, a balloon is, to all intents and purposes, a part of the atmosphere. There is scarcely any sensation of movement, either vertically or horizontally. The earth appears to drop away from beneath and to sweep by horizontally, • and regardless of how violently the wind may be blowing, the balloon is always in a dead calm because it is really part of the wind itself and is traveling with it at exactly the same speed. If it were not for the loss of lifting power through the expansion and con traction of the gas, making it necessary to per mit its escape in order to avoid rising to incon venient heights on a very warm day, and the sacrifice of ballast to prevent coming to earth at night the ability of a balloon to stay up would be limited only to the endurance of its crew and the quantity of provisions it was able to transport. Unless a voyage is to be governed in its direction entirely by the wind, the dirigible must possess a means of moving contrary to the latter. The moment this is attempted resistance is encountered, and it is this resistance of the air that is responsible for the chief difficulties in the design of the diri gible. To drive it against the wind it must have power; to support the weight of the power plant the size of the gas bag must be increased. It is also necessary to select a form that presents as small a surface• as pos sible to the air as the dirigible advances, while preserving the maximum lifting power. Ex perience has strikingly demonstrated the anal ogy betwen the marine and aerial practice not only is the shape of the bow of the vessel of great importance but, likewise, the stern. The profile of the latter may permit of an easy reunion of the molecules of air separated by the former, or it may allow them to come together again suddenly, clashing with one another and producing disturbing eddies just behind the moving body. Marey-Monge laid
down the principle that to be successfully pro pelled through the air, the dirigible must have ethe head of a cod and the tail of a mackerel?' and nature exemplifies the truth of this in all swiftly moving fishes and birds. The pointed stern prevents the formation of eddies and the creation of a partial vacuum in the wake, which would impose additional thrust on the bow. Zeppelin has disregarded this factor by adher ing to the purely cylindrical form with short hemispherical bow and stern, but it is worth note that while other German investigators originally followed this precedent, they have gradually abandoned it, owing to the noticeable retarding effect.
Next in importance to the best form to be given the -vessel is the most effective size— something which has a direct bearing upon its lifting power. This depends upon the volume, while the resistance is proportional to the amount of surface presented. Greater lifting power can accordingly be obtained by keeping the diameter down and increasing the length. As the gas is frequently under considerable pressure when the balloon expands under the influence of the sun's heat, a great deal of experiment has been necessary to find the best class of fabric for the making of the en velope. Under the pressure an ordinary fabric would stretch and permit the escape of a large percentage of the gas. It has been found impossible to weave any fabric that will be close enough to hold hydrogen under pressure, so that recourse is had to a combination of cloth and rubber. The cloth is an extremely fine weave of cotton, even closer and lighter than the best of racing yacht duck, and is com bined with rubber under high pressures. Three layers of this rubberized fabric are cemented together to form what is known as ((balloon cloth;' which is about as impenetrable a material as can be made without involving undue weight. The necessity of using rubber in it has intro duced a complication, it having been found by experiment that rubber is strongly attacked by the ultra-violet rays of sunlight, which prob. ably accounts for the fact that balloon en velopes are usually found more or less damaged after a high ascension, the influence of these rays being much greater at higher altitudes.